A halftone image may be prepared by a conventional computer, laser printer, copier, or facsimile machine by digital computation and then may be stored in whole or in part prior to transferring the image to a tangible medium such as printable media, film, or a printing plate. A halftone image is composed of output picture elements (pixels). A halftone image is generally prepared to satisfy a limitation of the media or of the transfer process that prohibits the formation of an output pixel having an analog intensity level.
Preparation of a halftone image using the conventional clustered dot technique includes grouping output pixels to form a halftone dot that varies in size and proximity to other halftone dots to convey analog pixel intensity information (e.g., a shade of grey) that cannot be conveyed by varying output pixel intensity. Output pixels generally have a binary intensity (e.g., black when printed or white when not printed). Translation by digital computation usually introduces some quantization and possibly inaccurate variation of halftone dot size and placement. When a halftone image is transferred to tangible media, variation in halftone dot size and placement (such as introduced by a mechanical system) as well as any effects of quantization and inaccuracy may give rise to patterns generally known as moire patterns. For example, a large area of the same color in the original may have a pattern of objectionable variation in color.
In a conventional four color printing process, for example, halftone images prepared for each component color by known techniques can produce objectionable moire patterns when overlaid, one atop the other. These patterns are especially noticeable when transfer to media involves minor misregistration of the component halftone images. Because any of a large number of causes of misregistration often develop either during manufacture or during use of a conventional color printer, a need remains for systems and methods for preparing halftone images that are less prone to produce objectionable moire patterns upon minor misregistration.
Without an improved halftone preparation technique, comparatively inexpensive printing systems (e.g., having output pixel densities of a few hundred pixels per inch) cannot be used to supply accurate and moire-free graphic images on tangible media. The demand for such images is currently met in part by more expensive printing systems having output pixel densities of a few thousand pixels per inch. The demand for such images is expected to increase as the cost per image declines. These market demands intensify the need for systems that provide a halftone image and for methods of providing a halftone image.